This application is related to Japanese Patent Application No. HEI 11(1999)-295470 filed on Oct. 18, 1999, whose priority is claimed under 35 USC xc2xa7 919, the disclosure of which is incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates to a surface acoustic wave (SAW) device and a process for manufacturing the same. More particularly, the present invention relates to a SAW device having an improved power durability and its manufacture process. The surface acoustic wave device of the present invention can be suitably used as a SAW device (a resonator, an interstage filter, a duplexer or the like) in mobile communication terminals such as automobile phones and portable phones.
2. Description of Related Art
Recently, development of smaller and less heavy mobile communication terminals such as automobile phones and portable phones has been rapidly taking place. With this development, there is an increasing need for smaller and more efficient parts for the mobile communication terminals, giving rise to the need also for surface acoustic wave (SAW) devices that contribute to miniaturization of high frequency (RF) sections.
Among the duplexers, the antenna duplexer is typically located at the front end section of the RF section and requires large power durability. Here, since conventional SAW devices do not have a sufficient power durability, dielectric filters have been used. Having a large size, however, the dielectric filters have been an obstacle to miniaturization.
Generally, the SAW device is comprised of interdigital transducers on a piezoelectric single-crystal substrate of LiNbO3, LiTaO3, rock crystal or the like. Al (aluminum) and Al alloys which have excellent characteristics such as low resistance, light weight and good micro-workability are widely used for the interdigital transducers. When a high-power high-frequency signal is applied to the SAW device, electromigration caused by high-frequency current and mechanical migration caused by displacement of a SAW are liable to occur in Al of the interdigital transducers. The interdigital transducers are thus deteriorated, and as a result, the characteristics of the SAW device are also deteriorated, as those skilled in the art know.
For improving the power durability of the SAW device, electrodes formed of the following materials have been proposed:
(1) Electrodes of an Al alloy consisting of two elements, that is, Al and another element:
Al-0.1 to 0.3 wt % Pd (Japanese Unexamined Patent Publication No. HEI 2(1990)-274008)
Al-0.01 to 5 wt % Sc (Japanese Unexamined Patent Publication No. HEI 5(1993)-183378)
(2) Electrodes of an Al alloy consisting of three or more elements, that is, Al and other two or more elements:
Alxe2x80x94Cuxe2x80x94Mg (Japanese Unexamined Patent Publication No. SHO 64(1989)-80113)
Al-additive A-additive B (additive A is a metal that forms solid solution with Al (Sc, Ga, Hf, Zn, Mg), additive B is a metal that forms an intermetallic compound with Al (Ge, Cu, Si)) (Japanese Unexamined Patent Publication No. HEI 10(1998)-22764)
Since desired characteristics cannot be realized with the above-mentioned single-layer electrodes, the following electrodes consisting of laminated films have been proposed:
a laminate of an Alxe2x80x94Cu film and a Cu film (Japanese Unexamined Patent Publication No. HEI 7(1995)-122961)
This publication describes that CuAl2 formed at a grain boundary of the Alxe2x80x94Cu film and at an interface between the Alxe2x80x94Cu film and the Cu film can prevent migrations.
a laminate of an Alxe2x80x94Cu film (lower layer) and an Alxe2x80x94Scxe2x80x94Cu film (upper layer) (Japanese Unexamined Patent Publication No. HEI 8(1996)-330892)
This publication describes that the use of the Alxe2x80x94Scxe2x80x94Cu film for the upper layer which has a large stress and the Alxe2x80x94Cu film for the lower layer which has a higher current density can achieve both improvement in the power durability and reduction in signal loss.
laminates of an Al or Al alloy film (upper layer) and films of various kinds (lower layer) (Japanese Unexamined Patent Publication Nos. HEI 5(1993)-90268, HEI 5(1993)-226337, HEI 7(1995)-135443, HEI 8(1996)-340233)
These publications describe that the orientation properties of the Al or Al alloy film is improved by selecting a material for the lower layer and thereby a migration resistance is enhanced.
a laminate of an Al or Al alloy film and a film of a metal having a larger coefficient of diffusion into Al than a coefficient of self diffusion of Al (Japanese Unexamined Patent Publication No. HEI 9(1997)-223944)
This publication describes that the power durability is improved by an alloy layer or solid solution layer formed by mutual diffusion of the components of the two films.
However, none of the above-mentioned constructions can provide sufficient characteristics.
The inventors of the present invention have found that the power durability is improved by laminated Al-3 to 10 wt % Mg film and Al-0 to 5 wt % X film (X is selected from the group consisting of Cu, Ti, Pd, gi and Ge) (see Japanese Unexamined Patent Publication No. HEI 10(1998)-135767). With this construction, a good power durability can be achieved by a solid solution enhancement of the Alxe2x80x94Mg alloy.
However, with an increasing need for further miniaturization of SAW device and also for higher-frequency SAW devices, the electrodes of the SAW devices are becoming smaller and further improvement of the power durability is desired, for example, for application as a quasi-microwave band filter.
Accordingly, the present invention provides a surface acoustic wave device including a substrate at least a surface of which has a piezoelectric function, and an electrode formed on the substrate, wherein the electrode is composed of a first film of Al containing Cu at or over a solid solubility limit of Cu to Al and one or more unit(s) of a second film of Al containing Mg at or over a solid solubility limit of Mg to Al and a third film of Al containing Cu at or over the solid solubility limit of Cu to Al formed on the first film in this order, and the first film and the third film contain Mg diffused from the second film.
The present invention also provides a process for manufacturing a surface acoustic wave device including a substrate at least a surface of which has a piezoelectric function and an electrode formed on the substrate, the process including the steps of forming a first film of Al containing Cu at or over the solid solubility limit of Cu to Al on the substrate, forming one or more unit(s) of a second film of Al containing Mg at or over the solid solubility limit of Mg to Al and a third film of Al containing Cu at or over the solid solubility limit of Cu to Al on the first film in this order, and thermally treating the first, second and third films at a temperature of 100 to 350xc2x0 C. so as to diffuse Mg from the second film to the first and third films, thereby forming the electrode.
These and other objects of the present application will become more readily apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.